The Genesis of Ultramafic Rock Mass on the Northern Slope of Lüliang Mountain in North Qaidam, China

IF 2.2 4区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Minerals Pub Date : 2024-08-27 DOI:10.3390/min14090871
Haiming Guo, Yanguang Li, Bo Chen, Huishan Zhang, Xiaoyong Yang, Li He, Yongjiu Ma, Yunping Li, Jincheng Luo, Haichao Zhao
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Abstract

The ultramafic rock located on the northern slope of Lüliang Mountain in the northwestern region of North Qaidam Orogen is altered to serpentinite. The occurrence of disseminated chromite within the serpentinite holds significant implications for understanding the petrogenesis of the protolith. This work provides strong evidence of a distinct zonal texture in the chromite found in the ultramafic rock, using petrographic microstructure and electron probe composition analysis. The core of the chromite is characterized by high contents of Cr#, with enrichment in Fe3+# (Fe3+/(Cr + Al + Fe3+)) and depletion in Al2O3 and TiO2. The Cr2O3 content ranges from 51.64% to 53.72%, while the Cr# values range from 0.80 to 0.84. The FeO content varies from 24.9% to 27.8%, while the Fe2O3 content ranges from 5.19% to 8.74%. The Al2O3 content ranges from 6.70% to 9.20%, and the TiO2 content is below the detection limit (<0.1%). Furthermore, the rocks exhibit Mg# values ranging from 0.13 to 0.25 and Fe3+# values ranging from 0.07 to 0.12. The mineral chemistry of the chromite core in the ultramafic rock suggests it to be from an ophiolite. This ophiolite originated from the fore-arc deficit asthenosphere in a supra-subduction zone. The estimated average crystallization temperature and pressure of the chromite are 1306.02 °C and 3.41 GPa, respectively. These values suggest that the chromite formed at a depth of approximately 110 km, which is comparable to that of the asthenosphere. The chromite grains are surrounded by thick rims composed of Cr-rich magnetite characterized by enrichment in Fe3+# contents and depletions in Cr2O3, Al2O3, TiO2, and Cr#. The FeO content ranges from 28.25% to 31.15%, while the Fe2O3 content ranges from 44.94% to 68.92%. The Cr2O3 content ranges from 0.18% to 23.59%, and the Al2O3 and TiO2 contents are below the detection limit (<0.1%). Moreover, the rim of the Cr-rich magnetite exhibits Cr# values ranging from 0.90 to 1.00, Mg# values ranging from 0.01 to 0.06, and Fe3+# values ranging from 0.64 to 1.00, indicating late-stage alteration processes. The LA-ICP-MS zircon U-Pb dating of the ultramafic rock yielded an age of 480.6 ± 2.4 Ma (MSWD = 0.46, n = 18), representing the crystallization age of the ultramafic rock. This evidence suggests that the host rock of chromite is an ultramafic cumulate, which is part of the ophiolite suite. It originated from the fore-arc deficit asthenosphere in a supra-subduction zone during the northward subduction of the North Qaidam Ocean in the Ordovician period. Furthermore, clear evidence of Fe-hydrothermal alteration during the post-uplift-denudation stage is observed.
中国北柴达木吕梁山北坡超基性岩体的成因
位于北柴达木造山带西北部吕梁山北坡的超基性岩蚀变为蛇纹岩。蛇绿岩中出现的弥散铬铁矿对了解原岩的岩石成因具有重要意义。这项研究利用岩石学显微结构和电子探针成分分析,提供了在超基性岩中发现的铬铁矿具有明显带状纹理的有力证据。铬铁矿的核心特征是 Cr# 含量高,Fe3+#(Fe3+/(Cr + Al + Fe3+))富集,Al2O3 和 TiO2 贫化。Cr2O3 含量在 51.64% 至 53.72% 之间,而 Cr# 值在 0.80 至 0.84 之间。FeO 含量从 24.9% 到 27.8%,Fe2O3 含量从 5.19% 到 8.74%。Al2O3 含量介于 6.70% 至 9.20% 之间,TiO2 含量低于检测限(<0.1%)。此外,岩石的 Mg# 值介于 0.13 至 0.25 之间,Fe3+# 值介于 0.07 至 0.12 之间。超基性岩中铬铁矿核心的矿物化学成分表明,它来自蛇绿岩。该蛇绿岩起源于超俯冲带的前弧亏缺星体层。铬铁矿的估计平均结晶温度和压力分别为1306.02 ℃和3.41 GPa。这些数值表明铬铁矿形成于大约110千米的深度,与岩石圈的深度相当。铬铁矿晶粒周围环绕着由富铬磁铁矿组成的厚边缘,其特征是Fe3+#含量丰富,而Cr2O3、Al2O3、TiO2和Cr#含量贫乏。FeO 含量在 28.25% 到 31.15% 之间,而 Fe2O3 含量在 44.94% 到 68.92% 之间。Cr2O3 含量为 0.18% 至 23.59%,Al2O3 和 TiO2 含量低于检测限(<0.1%)。此外,富铬磁铁矿边缘的 Cr# 值介于 0.90 至 1.00 之间,Mg# 值介于 0.01 至 0.06 之间,Fe3+# 值介于 0.64 至 1.00 之间,表明晚期蚀变过程。超基性岩的 LA-ICP-MS 锆石 U-Pb 定年结果为 480.6 ± 2.4 Ma(MSWD = 0.46,n = 18),代表超基性岩的结晶年龄。这些证据表明,铬铁矿的母岩是属于蛇绿岩组的超基性积岩。它起源于奥陶纪北柴达木洋向北俯冲时期超俯冲带的前弧亏损星体。此外,还观察到在上升-沉积后阶段发生铁-热液蚀变的明显证据。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Minerals
Minerals MINERALOGY-MINING & MINERAL PROCESSING
CiteScore
4.10
自引率
20.00%
发文量
1351
审稿时长
19.04 days
期刊介绍: Minerals (ISSN 2075-163X) is an international open access journal that covers the broad field of mineralogy, economic mineral resources, mineral exploration, innovative mining techniques and advances in mineral processing. It publishes reviews, regular research papers and short notes. Our aim is to encourage scientists to publish their experimental and theoretical results in as much detail as possible. There is no restriction on the length of the papers. The full experimental details must be provided so that the results can be reproduced.
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